The present invention relates to a head switching control apparatus and method for use in a video cassette recorder having at least two heads, and more particularly to an automatic head switching control method and apparatus which detects whether a delay time between a head switching point of time and a vertical sync signal is within a predetermined range, and generates a head switching signal according to head switching delay data in order to control a head switching operation.
The head switching control is necessarily needed to maintain a uniform relative position between a video head attached to a drum and a video signal recorded on a tape, when one frame is formed of signals picked up by a plurality of heads.
The head switching point of time is determined by the head switching signal. In a VHS system, the head switching point of time is produced at an interval of time prior to a rising edge of a vertical sync signal of a video signal ranging between five horizontal sync intervals (5H) and eight horizontal sync intervals (8H).
FIG. 1 is a waveform timing diagram showing a process of producing a head switching signal in the VHS system.
FIG. 1 shows a drum phase generation DPG signal (wave form A) and a drum frequency generation DFG signal (waveform B), which are generated from a head drum, respectively. Waveform C of FIG. 1 shows a head switching signal for determining a head switching point of time. Waveform D of FIG. 1 shows a vertical sync signal of a video signal. In the VHS system, the head switching point of time of the head switching signal should lead the vertical sync of the video signal by 5H through 8H centered at 6.5H. As shown by waveforms C and D in FIG. 1, the head switching point of time leads the vertical sync signal by 6.5H.
An interval t.sub.H'D for determining the head switching point of time is determined by a time t.sub.fg from a falling edge of the DPG signal to a first rising edge of the DFG signal and a delay time t.sub.D which is determined by head switching delay data which is controlled by a controller.
The relationship between the head switching delay data (hereinafter referred to as the delay data) and the delay time follows an equation (1). ##EQU1##
Here, t.sub.D is a delay time, N is delay data of which the unit is one byte (8 bits), and f.sub.sc is a color subcarrier sync frequency.
Since time t.sub.fg is a fixed value in a drum system, an automatic head switching control represents that the delay data varies stepwisely and delay time t.sub.D is controlled by the controller, in order to match a head switching point of time.
FIG. 2 is a block diagram of a conventional automatic head switching control apparatus.
In FIG. 2, a video processor 1 includes a pre-amplifier (not shown). The pre-amplifier detects an envelope state, that is, an amount of a video signal which is currently picked up by the video heads, and supplies the detected information to controller 2. Controller 2 generates delay data for altering delay time t.sub.D according to the envelope detection signal. The output end of controller 2 is connected to a servo portion 3. A head switching signal is determined by signals DPG and DFG applied from a drum 4 and the delay data output from controller 2. Servo portion 3 supplies a control signal to a motor in drum 4 according to the head switching signal. The controlled drum motor rotates in drum 4 and a video signal is read out from a recording medium by the video heads attached to the drum. The read video signal is supplied to video processor 1 to detect a new envelope state. Such an operation repeats while altering the delay data, until the envelope detection signal becomes a high-level state. When a high-level envelope detection signal is applied to controller 2 from video processor 1, the delay data at this time is stored in controller 2.
However, the above conventional automatic head switching control method has a problem. The interval between the head switching point of time and the vertical sync signal cannot be maintained invariably in the VHS system when the pre-amplifier does not have an accurate envelope detection performance. Since the drum motor is controlled every time with respect to the delay data varying in the controller, it is another problem that it takes a considerable time to control a head switching operation due to a time taken until the velocity of the drum motor is stabilized.